The present invention relates to a method of recording information signals in digital form on a memory medium at a relatively high density or reproducing (or detecting) information signals previously recorded in digital form on a memory medium at a relatively high density, and a head device and a memory medium used for the recording/reproducing method.
Various kinds of large-capacity memories are presently known, for example, a semiconductor memory represented by a DRAM or Flash memory, a magnetic tape represented by a video recorder, and a disk memory represented by a compact disk or hard disk. These memories having problems, for example, in terms of high bit cost and low access speed may be not suitably used for the future information inputting/outputting apparatuses such as a microprocessor or network requiring the more increased data transfer rate and data capacity. A related art hard disk, optical disk or magnetic tape is lower two digits or more in cost per unit data (bit) than a semiconductor memory; however, it is significantly inferior in access time, data transfer time and volume of the disk to the semiconductor memory.
At present, with the enhanced performances of computers and the increased communication speed of information networks, the quantity of data to be processed has become larger and the processing rate of data to be processed has become high. To meet such technical development, it is desired to realize a read only memory and a writable memory with the cost per bit kept substantially comparable to that of a magnetic disk or optical disk and with the access time, data transfer time and volume of the memory increased to the levels comparable to those of a semiconductor memory.
The size of a semiconductor chip, for example, used in a DRAM has become larger with the progressing technical generation, and it is expected that the size of a semiconductor chip be more than about 3.times.3 cm at the 4 Gbit-generation. In this case, the area including a package will be about 12 cm.sup.2. To be used like such a DRAM, a memory to be developed is desired to have a size smaller than the above value and a low bit cost.
The memory capacity stored in the above-described area (about 12 cm.sup.2) is preferably equivalent to a capacity, for example, which is capable of storing dynamic images for about one hour, and more specifically, the memory capacity is required to have about 12 Gbits, that is, a memory density of at least 1 Gbit/cm.sup.2 in consideration of digital image signals with the compressed frequency band.
As the memory capable of meeting the above-described requirement, there have been extensively studied memories of a type using a so-called SPM (Scanning Probe Microscope) such as a STM (Scanning Tunneling Microscope) or AFM (Atomic Force Microscope).
Such a memory has been described in detail, for example, in "H. J. Mamin et al.: IBM J. Res. Develop. Vol. 39, 681 (1995)". This memory detects an information signal using a head device 100 shown in FIG. 10A. The head device 100 has a beam cantilevered with its one end fixed on a head substrate 102, which is generally called a cantilever 103, and a head element 101 as a signal detecting portion (hereinafter, referred to simply as "head element") formed at the leading end of the cantilever 103. The head element 101 is sharply pointed into the shape of a triangular or quadrangular prism by a semiconductor process. The leading end of the head element 101 sharply pointed up to the level of atomic size is moved close to the surface of a substance to be measured (data surface in the case of the memory), and an interatomic force acting between the head element 101 and the surface of the substance or a tunneling current flowing therebetween is directly or indirectly measured, to thus obtain information therefrom.
B. D. Terris et al. have reported in "Appl. Phys. Lett. 69(27), 4262(1996)" a method of preparing a data patten applicable to a disk-like medium by an electron beam plotting apparatus, transferring the patten on an ultraviolet cured resin layer formed on a glass disk by a so-called glass 2P process to prepare a data disk, and reproducing data signals stored in the disk by an AFM.
H. J. Mamin et al. have reported in "Sensors and Actuators A48, 215(1995)" a method of bringing a leading end of an AFM in contact with a high polymer substrate, heating the leading end of the AFM by laser to melt the surface of the high polymer substrate, thereby recording data on the surface of the high polymer substrate, and reproducing the data at a reproducing rate as high as 1 Mbit/sec by the AFM.
The apparatus for recording/reproducing information signals in each of these documents carries out recording/reproducing with the disk medium rotated using one AFM head.
H. Kado et al. have reported in "Appl. Phys. Lett. 66(22), 2961(1995)" a method of forming a platinum thin film on a silicon substrate and also forming an amorphous GeSb.sub.2 Te.sub.4 film thereon, carrying out recording by applying a pulsive electric field between a sharpened conductive head element and the platinum thin film, and carrying out reproducing by detecting a difference in electric conductivity as a change in current.
The above detection of data using the AFM, however, is not suitable for reproducing information signals at a high rate because the interatomic force is converted into a mechanical displacement of the cantilever 103 and the displacement is detected by a displacement meter using a piezoelectric effect or laser. Also in the case where information signals are recorded or reproduced on or from a rotating disk-like recording medium, the above detection of data using the AFM is disadvantageous in that it takes a time to wait rotation and the access speed becomes low.
To improve an effective data transfer rate, there has been known a method of carrying out parallel processing using a plurality of head devices. For example, S. C. Minne et al. have reported in "Appl. Phys. Lett. 67(26), 3918(1995)" an apparatus in which two AFM head devices with leading ends of head elements separated 100 .mu.m from each other are arranged in parallel whereby images of a grating with a cycle of 5 .mu.m are reproduced. In this parallel processing apparatus, ZnO having a piezoelectric effect is used for part of each cantilever to individually displace the two cantilevers in the depth direction, and the size of the cantilever becomes larger (length: 420 .mu.m, width: 85 .mu.m) for sufficiently ensuring the displacement, with a result that the mechanical resonance frequency becomes low to reduce the data transfer rate. Accordingly, even in the case of using a plurality of the AFM head devices, the data transfer rate is not improved so much.
The data reproducing apparatus (microscope) using a plurality of the head elements described in the above document does not report a method or mechanism of detecting or correcting a positional relationship between each head element and desired data in the direction parallel to the surface to be measured, causing a problem that the address management for data which is important for the memory apparatus cannot be performed. Even if each distance and positional relationship between two pieces of respective head elements has been clearly measured and also the positional relationship between either one of the head elements and the address of the data surface has been measured, the relationship between each head element and the data position cannot be kept resulting from a difference in thermal expansion coefficient between the head array and the substrate of the recording medium, for example, caused by temperature change.
In the memory apparatus using the SPM, since the leading end of the head element 101 of the head device 100 is very sharpened as shown in FIG. 10B and only the leading end of the head element 101 is brought in contact with the data surface, if an impact force is applied to the memory apparatus during reproducing of data, the data surface in contact with the leading end of the head element 101 may be applied with a very high local pressure, which causes a fear of breakage of data stored in the data surface. For example, S. C. Minne et al. have reported in "Sensors and Actuators A48, 215 (1955)" that a leading end of a head element is formed into a spherical shape having a curvature of 100 nm or less. A spring constant of the cantilever is in the order of 1 N/m. Now, it is assumed that a leading end of a head element applied with an impact force is displaced 10 nm on the data surface side and is brought in contact with the data surface; and the leading end of the head element is formed into a flat circular shape having a radius of 10 nm. In this case, a pressure applied to the flat circle portion is as very high as 3.times.10.sup.7 N/m.sup.2, which will cause breakage data stored on the data surface of a medium, insofar as the medium is made from a usual material. Even if there does not occur breakage of data, since the leading end of the head element is worn, there arises another problem that the shape of the leading end is changed and thereby the resolution in recording or reproducing is reduced.
In the above-described recording/reproducing method proposed by H. Kado, since a current value upon reproducing is 10 pA at a location where information signals have been not recorded and is 1 nA at a location where information signals have been recorded, a reproducing signal with a sufficient S/N can be obtained when the data reproducing rate is low because the frequency band of the reproducing signal is narrow; however, a reproducing signal with a sufficient S/N cannot be ensured at the above small signal current of about 1 nA when the data reproducing rate is high because the frequency band of the reproducing signal becomes wide.